Dual three-spots optical scanning device

ABSTRACT

A radiation detector array for radial tracking error detection for a dual-wavelength optical scanning device. Three spot detectors ( 100, 102, 104 ) for conducting three spot radial tracking error detection at each of the two different wavelengths. Satellite spot detectors ( 102, 104 ) are divided into three detector elements and one switched depending on the wavelength currently being used for scanning.

This invention relates to a dual optical scanning device for scanningoptical record carriers with any one of two different wavelengths, thedevice comprising a radiation detector array for performing scanningerror detection, and to such a radiation detector array.

Currently, scanning devices using two-wavelength laser devices are beingstudied. Typically, the scanning devices use a common beam path for bothwavelengths and the two wavelengths are joined after emission fromdifferent parts of the laser device, for example by means of adiffraction grating. There is a need to perform scanning error detectionfor both wavelengths.

Typical scanning error detection methods used in optical disk scanningdevices are focus error detection and tracking error detection. Variousdifferent methods are known for focus error detection and radialtracking error detection. The focus error detection methods includeknife edge pupil obscuration, in which the beam is split into two bye.g. a prism and the position of the spots on two spot detectorsindicate correct focusing; astigmatic focusing, in which an astigmaticspot on the detector is created by means of a cylindrical lens or aplane parallel plate, and variations in the shape of the spot fromcircular are detected by a diamond-shaped quadrant spot detector; andspot size detection, in which the beam is separated into two by e.g. amicroprism and detecting the resulting spot sizes before and afterrefocusing respectively.

Radial tracking error detection methods include push-pull radialtracking, in which a difference in signal between two pupil halves aremeasured on separate detectors; three spot (or three beam) centralaperture radial tracking, in which the radiation beam is split intothree by a diffraction grating, and the outer (satellite) spots are seta quarter track pitch off the main spot and the difference of theirsignals used to generate the tracking error signal; three spotspush-pull radial tracking, in which the radiation beam is split intothree by a diffraction grating and a difference between the push-pullsignals of the main spot and the satellite spots is used as the trackingerror signal; and Differential Phase or Time Detection (DPD or DTD)radial tracking, in which the contribution of the radial offset of thephase of the (±1, ±1) orders is exploited in a square-shaped quadrantspot detector. The three spot push-pull radial tracking system has theadvantage over one spot push-pull systems is that systematic errors,including symmetric errors and asymmetric errors, may be compensated forautomatically. However, the system requires additional detector elementsand connections, increasing the complexity of the detector array.

European patent application EP-A-0860819 describes an optical scanningdevice which uses two lasers with different wavelengths and a commonobjective lens to produce spots suitable for reading low density as wellas high density disks. Various different detector array arrangements areproposed for detecting focus error and tracking error during scanning.In one embodiment, two separate detector arrays are used for eachseparate wavelength. In another embodiment, a single detector array isused for each wavelength. The array includes two detector elements forthree beam tracking error detection at the longer of the wavelengths,whereas single beam tracking is used at the shorter wavelength.

If in scanning device, which uses two wavelengths a single detectorarray is used for push-pull tracking error detection, the problem arisesthat if the spacing between the n^(th) order spot detectors is correctfor one of the wavelength, these detectors can not be used to detect then^(th) order spots for the other wavelength with sufficient accuracy.

It is an object of the present invention to provide a solution for thisproblem. In accordance with one aspect of the present invention there isprovided a radiation detector array for radial tracking error detectionwhen scanning optical record carriers with two wavelengths, said arraycomprising a plurality of spot detectors for detecting first and secondgroups of radiation beams forming respectively first and second sets ofspots corresponding to different diffractive orders including a zerothorder and plus and minus n^(th) order, n being an integer of 1 or more,each said spot detector being arranged to detect a characteristic of aspot formed by a said beam and each said spot detector comprising aplurality of detector elements for detecting different parts of a saidspot, said array comprising a zeroth order spot detector arrangedsubstantially centrally and n^(th) order spot detectors arranged to eachside thereof, wherein said n^(th) order spot detectors are arranged toperform radial tracking error detection for a first set of spots inwhich the n^(th) order spots have a first predetermined spacingcharacteristic with respect to the zeroth order spot and for a secondset of spots in which the n^(th) order spots have a second, differentpredetermined spacing characteristic with respect to the zeroth orderspot.

In accordance with a second aspect of the invention, there is provided adual optical scanning device, using two wavelengths, comprising aradiation detector array as described.

Further aspects and advantages of the invention will become apparentfrom the following description of preferred embodiments of theinvention, made with reference to the accompanying drawings, wherein:

FIG. 1 is a schematic plan view of an optical scanning device arrangedin accordance with embodiments of the invention;

FIG. 2 is a schematic plan views of three spots focused on conventionaldata tracks of an optical disk;

FIG. 3 is a schematic plan view of a conventional three spots push-pulltracking error detector array; and

FIGS. 4 and 5 show a schematic plan view of a detector array arranged inaccordance with an embodiment of the invention;

In accordance with an embodiment of the invention, at least two formatsof optical disk OD, such as the CDR(W) format and/or the DVD-RAM formatare used for storing data. The CDR(W) format disk may be written anddisks of both formats may be read-out by means of the optical scanningdevice. The disk includes an outer transparent layer covering at leastone information layer. In the case of a multilayer optical disk, two ormore information layers are arranged behind the cover layer, atdifferent depths within the disk. The side of the information layer, orin the case of a multilayer disk the side of the layer furthest awayfrom the cover layer, facing away from the transparent layer isprotected from environmental influences by a protection layer. The sideof the transparent layer facing the device is the disk entrance face.

Information may be stored in the information layer or layers of theoptical disk in the form of optically detectable marks arranged insubstantially parallel, concentric or spiral tracks. The marks may be inany optically readable form, for example in the form of pits or areaswith a reflection coefficient different from their surroundings. Theinformation layer or layers may be formed of an optically recordablematerial.

As shown in the embodiment of FIG. 1, the optical scanning deviceincludes a dual-wavelength radiation source 2, including for example twosemiconductor lasers, operating at two predetermined wavelengths, λ₁ andλ₂, for example λ₁=780 nm and λ₂=655 nm. The two lasers may beintegrated on one substrate. Source 2 selectively emits a divergingradiation beam of one of the two wavelengths. The light path includes apath joining component 4 with the function of joining the beam paths forthe two wavelengths. The joining component 4 may take the form of adiffractive element grating or a holographic element. In the case of aholographic element, the joining component 4 may also provide apre-collimator function. This function is needed for obtainingsufficient intensity for the write mode and sufficient rim intensity forreading a DVD format disk. A diffraction grating element 6 is used forforming three separate beams, including a main, zeroth order, beam andtwo, first order, satellite beams, for performing three spots push-pullradial tracking. A beam splitter 8 directs the incident beam towards afolding mirror 10. Behind the folding mirror 10 lies a collimating lens12. Before reaching the disk, the beams pass through an objective lens14 for focusing the beams onto spots on an information layer in the diskOD. The objective lens 14 is adapted to provide spherical aberrationcorrection for different substrate thickness of CD and DVD format disks,when being scanned using either of the two wavelengths λ₁ and λ₂.

On reflection from the disk, the beams are returned along the incidentbeam path until reaching beam splitter 8, which transmits the reflectedbeams. The beams are directed via detector lens 16 onto a photodetectorarray 18, which converts the optical signals into electrical signals fordata read-out, focus error control and tracking error control, as willbe described in further detail below. Forward sense photodiode 20 isused for accurately controlling the power of radiation source 2 during ascanning process, in particular during a writing process.

FIG. 2 shows an arrangement of three beams, namely first order satellitebeams a and b and zeroth order beam c, formed by grating 6, correctlytracking tracks of the optical disk OD.

FIG. 3 shows a conventional arrangement of three spot detectors, firstorder spot detectors a and b each including two half detector elements,a1, a2; b1, b2, and zeroth order spot detector c including four quadrantdetector elements c1, c2, c3, c4 respectively, used for detecting apush-pull radial tracking error in the three beam spots a, b and c andastigmatic focus error in the main beam spot c. The spot detectors a, b,c are arranged in the optical scanning device in a generally tangentialequivalent (track-parallel) direction. Three spots push pull radialtracking uses the push pull signal of all three spots. The push pullsignal of the main spot c and the two satellites a and b are describedas a function of the detracking x as:PP(c)=γm _(pp).sin(2πx/q)PP(a)=m _(pp).sin(2π(x−x ₀)/q)PP(b)=m _(pp).sin(2π(x+x ₀)/q)

In the above, m_(pp) is the push pull modulation, q is the track pitch,x₀ is the ideal separation of each of the spots a and b from the centralspot, generally set at q/2, by selection of the diffraction gratingpitch, to maximise the signal, and γ being the diffraction efficiency,or more particularly in the case of a grating, the grating ratio.

Connections are formed in the conventional detector array to provide theradial error signal (RE) as follows:RE=c 1 −c 2 −c 3+c 4−γ(a 1−a 2+b 1−b 2)

FIGS. 4 and 5 show a radiation detector array in accordance with anembodiment of the invention. The detectors are in the form of photodiodeelements forming separate spot detectors, each spot detector beingseparated into detector elements separated by separation lines providingdesired signal separations.

The arrangement in this embodiment includes three spot detectors 100,102, 104 arranged generally in a line, which is in a substantiallytrack-tangential equivalent direction in the director array. A centralspot detector 100 includes four rectangular detector elements C1 to C4,arranged side by side in a quadrant and separated by perpendicularseparation lines, to detect the location and shape of a main, zerothorder spot. Satellite spot detectors 102 and 104 each comprise threedetector elements, A1, A2, A3 and C1, C2, C3 respectively, separated bytwo separation lines arranged in the track-tangential equivalentdirection. Detectors 102, 104 detect first order satellite spots. Spotdetectors 100, 102, 104 are arranged to conduct three spot push pullradial tracking error and astigmatic focus error detection in a similarmanner as that described in the prior art, but for each of two groups ofbeams of the first and second wavelength, λ₁ and λ₂, respectively.

All detector elements of all spot detectors supply an output signal andthese signal are supplied to an electronic processing circuit, whereinthe output signals are combined and processed to a read-out signal, afocus error signal and a tracking error signal.

Connections are formed in the detector array of this embodiment toprovide a radial error signal (RE) as follows:For λ₁ RE=C 1 −C 2 −C 3+C 4−γ1(A 1+A 2−A 3+B 1+B 2−B 3)For λ₂ RE=C 1−C 2−C 3+C 4−γ2(A 1−A 2−A 3+B 1−B 2−B 3)

In the above, γ1 and γ2 are the grating ratios (power ratio main spotw.r.t satellite) for the two wavelengths. They both depend on the depthof the profile of the three spots grating 6. The tracking errorprocessing circuitry is adapted to compensate for the feature thattypically γ1 is not equal to γ2.

Note that in the processing circuit the output signals of detectorelements A2 and A3 and those of elements B1 and B2 are added for λ₁,whilst for λ2 the output signals of detector elements A1 and A2 andthose of elements B2 and B3 are added.

Note that only one three spots grating 6 is used for generating thegroups of beams at each of the two wavelengths. A different diffractionangle is created at the different wavelengths. Because of the differencein wavelength the correct distance(s) of the main spot to the satellitespots on the disk and the detector array is different for λ₁ and λ₂, asfollows:${s\left( \lambda_{1} \right)} = {\frac{\lambda_{1}}{\lambda_{2}} \cdot {s\left( \lambda_{2} \right)}}$

Note that the satellite spot detectors 102, 104 are split into threeparts, rather than the conventional two part satellite spot detectorsused for three spots radial push-pull tracking error detection. Allthree elements of the two detectors A1, A2, A3 and B1, B2, B3 are usedto detect the radial tracking error signal at each of the twowavelengths. However, the output from the central detector elements A2and B2 is switched in dependence on the wavelength currently being usedfor scanning.

FIG. 4 illustrates the positioning of the zeroth order spot and the twofirst order spots on the detector elements 100, 102, 104 when the firstwavelength λ₁ (the longer wavelength) is used for scanning, and whentracking is correct. In this correct tracking configuration, thesatellite spots are each respectively centred on the separation linesbetween the central detector elements A2 and B2 and the outer detectorelements A1 and B3 furthest away from the zeroth order spot detector100. The zeroth order spot, meanwhile is centred on the centralseparation line separating detector elements C1 and C2 and detectorelements C4 and C3, respectively. The distance between this centralseparation line in zeroth order detector element 100 and the outermostseparation lines in the satellite spot detectors 102, 104, is set at thecorrect spot separation at the first wavelength, s(λ₁), as shown in FIG.4.

FIG. 5 illustrates the positioning of the zeroth order spot and the twofirst order spots on the detector elements 100, 102, 104 when the secondwavelength λ₂ is used for scanning, and when tracking is correct. Inthis correct tracking configuration, the satellite spots are eachrespectively centred on the separation lines between the centraldetector elements A2 and B2 and the outer detector elements A3 and B1closest to the zeroth order spot detector 100. The zeroth order spot,meanwhile is centred on the central separation line separating detectorelements C1 and C2 and detector elements C4 and C3, respectively. Thedistance between this central separation line in zeroth order detectorelement 100 and the outermost separation lines in the satellite spotdetectors 102, 104, is set at the correct spot separation at the secondwavelength, s(λ₂), as shown in FIG. 5.

Note that the central detector element A2, B2 of each of the satellitespot detectors 102, 104 is of a smaller area than each of the outwarddetector elements A1, A2 and B1, B3. This is because the wavelengthvariation (from 780 (λ₁) to 655 (λ₂) nm) is relatively small; if agreater wavelength variation is employed, this may not be the case.

Note that the optimum spot separation for CDR(W), thus for λ₁, is 0.8μm, whilst for DVD-RAM, thus for λ₂ the optimum spot separation is 0.74μm.

In one embodiment the distance of the satellite spots w.r.t the track isadjusted in such a way that it is optimal for 650 nm, i.e. s(λ₁)=0.88 μmand s(λ₂)=0.74 μm (±0.2 μm) because this is most critical for DVD. Atypical set of signal levels, compared to the optimum, is as follows:CDR(W) 97% DVD-RAM 100%

In another embodiment, the distance is adjusted between the optimum forDVD and CD i.e. s(λ₁)=0.84 μm (±0.2 μm) and s(λ₂)=0.70 μm (±0.2 μm). Inthis embodiment the signal levels are as follows: CDR(W) 99% DVD-RAM 99%

Note that, in each of these embodiments, the correct tracking spotsdistance ratio, and hence the detector spacing ratio s(λ₁):s(λ₂) is setat approximately 780:655.

The invention is applicable to DVD/CDR(W) combined scanning devices,DVD-ROM/CD combined scanning devices, for DVD-RAM/CDR(W) Double Writerscanning devices, and to various combinations thereof.

The above embodiments are to be understood as illustrative examples ofthe invention. Further embodiments of the invention are envisaged. Forexample, instead of or in addition to detecting the first ordersatellite spots, second order satellite spots may be detected usingdetector elements similar to detector elements 102 and 104. Furthermore,the zeroth order detector may be arranged to conduct spot size focuserror detection instead of astigmatic focus error detection. It is to beunderstood that any feature described in relation to one embodiment mayalso be used in other of the embodiments. Furthermore, equivalents andmodifications not described above may also be employed without departingfrom the scope of the invention, which is defined in the accompanyingclaims.

1. A radiation detector array for radial tracking error detection whenscanning optical record carrier with any one of two differentwavelengths, said array comprising a plurality of spot detectors fordetecting first and second groups of radiation beams formingrespectively first and second sets of spots corresponding to differentdiffractive orders including a zeroth order and an n^(th) order, n beingan integer of 1 or more, each said spot detector being arranged todetect a characteristic of a spot formed by a said beam and each saidspot detector comprising a plurality of detector elements for detectingdifferent parts of a said spot, said array comprising a zeroth orderspot detector arranged substantially centrally and n^(th) order spotdetectors arranged to each side thereof, characterized in that saidn^(th) order spot detectors are arranged to perform radial trackingerror detection for a first set of spots in which the n^(th) order spotshave a first predetermined spacing characteristic with respect to thezeroth order spot and for a second set of spots in which the n^(th)order spots have a second, different predetermined spacingcharacteristic with respect to the zeroth order spot.
 2. A radiationdetector array according to claim 1, wherein said n^(th) order spotdetectors are arranged to perform push-pull radial tracking errordetection.
 3. A radiation detector array according to claim 1, whereinsaid n^(th) order spot detectors each comprise a plurality of detectorelements separated by separation means, said separation means comprisinga first separation means defining said first spacing characteristic anda second separation means defining said second spacing characteristic.4. A radiation detector array according to claim 3, wherein said firstseparation means and said second separation means are arranged to eachside of a central detector element used for detecting both said firstset of n^(th) order spots and said second set of n^(th) order spots. 5.A radiation detector array according to claim 4, wherein said n^(th)order spot detectors each comprise three detector elements, includingsaid central detector element and outer detector elements arranged toeach side thereof.
 6. A radiation detector array according to claim 5,wherein all three detector elements are used for detecting both saidfirst set of n^(th) order spots and said second set of n^(th) orderspots.
 7. A radiation detector array according to claim 5, wherein saiddetector elements have detecting surface widths measured perpendicularto said separation means, and wherein said outer detector elements eachhave a greater detecting surface width than said central detectorelement.
 8. A radiation detector array according to claim 1, whereinsaid zeroth order spot detector is arranged to detect a push pull radialtracking error for both said first set of spots and said second set ofspots.
 9. A radiation detector array according to claim 1, wherein saidzeroth order spot detector is arranged to detect a focus error for bothsaid first set of spots and said second set of spots.
 10. A radiationdetector array according to claim 1, wherein said first and secondspacing characteristics have a ratio of approximately 780:655.
 11. Adual optical scanning device, using two wavelengths, comprising aradiation detector array according to claim
 1. 12. A dual opticalscanning device according to claim 11, said device comprising adiffraction component for generating both said first group of radiationbeams and said second group of radiation beams.
 13. A dual opticalscanning device according to claim 11, comprising radiation source meansfor generating radiation of a first predetermined wavelength, of whichsaid first group of radiation beams is formed, and a secondpredetermined wavelength, of which said second group of radiation beamsis formed.
 14. An optical scanning device according to claim 13, whereinsaid first and second spacing characteristics have a ratio correspondingapproximately to the ratio of the first and second wavelengths.